Alternative means of exploiting coal which is otherwise not recoverable, such as underground coal gasification (UCG), will become much more attractive—and perhaps shortcomings such as groundwater contamination will be overlooked.

The study does contain a caveat: "new cycles could occur if a technological breakthrough allowed mining of coal from very thin seams or at much greater depths, or if non-producing coal districts become important producers."

I believe UCG is one of the wildcards. Massive deposits of deep, thin or undersea coal are not recoverable by conventional mining, but these could potentially be pyrolized in place and extracted as gas. With an estimated 3000 billion tons of coal off Norway and strong interest in Britain, coal could return as supplies of Russian natural gas taper off.

This does make it doubtful that carbon emissions would fall very far before rebounding. Nuclear is looking better and better every day.

Patzek's 2010 AAAS paper is available here. The charts on pages 20 and 21 are indicative of the very large amounts of coal that revised estimates put in Alaska and Siberia. The delays in production shown indicate the difficulties of mining and transport in those areas.